<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* .box{
            width: 200px;
            height: 200px;
            background-color: pink;
            clip-path: polygon(0 0,200px 0,0 200px);
        } */
        .box {
            width: 0;
            height: 0;
            border-style: solid;
            border-width: 0 50px 50px;
            border-color: transparent transparent #d9534f;
        }
        .box::before{

        }
    </style>
</head>

<body>
    <div class="box">

    </div>
</body>

</html>